Ultrathin thermally grown silicon dioxide and silicon oxynitride layers with thicknesses between 3 and 6 nm are characterised by spectroscopic ellipsometry, The ellipsometric measurements are interpreted using one- and two-layer optical models and different Si dielectric function reference data available in the literature. It is shown that using the newest more accurate reference data gives not only an improved fit to the experimental data, but also a different, more realistic physical model for ultrathin oxide and oxynitride layers on silicon.